That’s still not exactly easy, but it is less complicated than asking a probe to fly all the way to Europa, safely land, burrow through a miles-thick crust of rock-hard ice, and then get to work being an extraterrestrial ocean explorer.

“It’s unlikely that one of these plumes is going to throw a fish into space that’s going to whack into Europa Clipper,” saysCynthia Phillips of NASA’s Jet Propulsion Laboratory. “It’s more likely to come from pockets of liquid that are closer to the surface – so, not free ocean samples, but free subsurface samples.”

In Bloom

For years, planetary scientists have argued over whether Europa might be spitting water into space, as Saturn’s moon Enceladus does.

View Images

An image of the icy plumes spouting from Saturn's moon Enceladus, as seen by NASA's Cassini spacecraft.

Photograph by NASA/JPL/Space Science Institute

In late 2013, tantalizing images from the Hubble Space Telescope revealed possible125-mile-high plumes of water vapor erupting from a region in the moon’s southern hemisphere. At the time, though, scientists were skeptical, as the plumes were at the very limit of Hubble’s ability to see. Follow-up observations revealed nothing.

But in 2016, andagain in 2017, scientists reported that more Hubble imagespointed to the presence of a plume, though something less dramatically exuberant than the geysers of Enceladus, which fly so high that they create a ring around Saturn. Even so, Europa’s eruptions could be equally as dense as its cousin’s, and easily visible by an orbiting spacecraft.

That’s when the University of Michigan’sXianzhe Jia and colleagues decided to revisit those archival data and see what kinds of nuggets they could find.

“I was asking myself, Why didn’t we start looking at this earlier? Why wait so long? The data is there, publicly available for almost 20 years,” Jia says.

Something in the Way

Europa’s gravity is strong enough that any erupted water vapor would hug the moon quite closely, and Galileo only swung low enough to detect such an eruption twice, most notably in December 1997. During that pass, the spacecraft took about five minutes to traverse Europa’s face.

Jia and his colleagues pulled observations from the spacecraft’s magnetometer, which measures magnetic fields, and from a second instrument that measures the density of charged particles. In the sequence of numbers produced by those two instruments, they immediately spotted something unusual: Anomalous blips, lasting about three minutes, centered around Galileo’s closest approach to the moon.

If a plume were erupting, Jia says, the erupted water vapor and dust particles would be affected by magnetic fields, which is what the spacecraft detected. And the density of charged particles surrounding the spacecraft would change as the spacecraft entered, flew through, and then exited the plume.

“We saw very peculiar changes in the magnetic signal, something I don’t think has been explained in the past,” Jia says. “We also pulled out the plasma wave data from Galileo, and surprisingly, around the same time, the plasma wave showed anomalous emissions. So when you put those two together, that indicated that something very special had happened during that interval.”

In other words, the spacecraft had flown through a localized plume maybe 620 miles wide somewhere near the moon’s equator. But Jia and his team wanted to make sure. So they simulated the observations such a spacecraft would make if it flew through a plume of the size and density spotted by Hubble. The Galileo observations matched the simulation almost perfectly.

Now, with two spacecraft and several independent instruments reporting similar findings, it’s harder to deny that Europa is venting water vapor into space.

“If you look at any one piece of evidence by itself, it’s not very convincing,” McGrath says. “But when you start bringing in completely independent sets of observations and they all seem to be telling us the same thing, it’s sort of like, yeah. That’s what is starting to convince people.“

Come As You Are

As exciting as the Galileo data are, they don’t necessarily prove the existence of continual plumes. Instead, Phillips says, the observation is another clue that can be added to a body of evidence suggesting that Europa does—at least occasionally—vent water into space.

Solar System 101How many planets are in the solar system? How did it form in the Milky Way galaxy? Learn facts about the solar system's genesis, plus its planets, moons, and asteroids.

“I think it tells you there’s probably more plumes than we can see right now, because the odds that we’d happen to fly through the only one that exists are pretty low,” McGrath says.

What does this mean for the Clipper spacecraft? Turns out, the team designing the probe has already planned to include a suite of instruments capable of tasting a plume, should the spacecraft fly through. What it finds will be anyone’s guess.

“Even with our wildest imagination, we always see stuff that we totally did not expect,” McGrath says. “We’ll surely see something we totally don’t expect at Europa.”